Mulch best way to amend soil, help plants survive

Here is a great opportunity to get involved in supporting local community gardens. Saturday from 5-8 p.m. is the Tonopah Community Gardens’ first Wine and Cheese Fundraiser. The garden is located at 711 N. Tonopah Drive. Come on out and take a look at a new community garden, have some refreshments and help a good cause.

Q: In your columns, you frequently refer to amending the soil to improve the prospects of trees and other plants in the Las Vegas Valley, due to our poor soils. However, I do not recall you explaining how to actually accomplish this for existing trees. If soil is amended by actually working it in, would this not risk damaging the root systems of small trees and bushes, not to mention the irrigation emitters and hoses?

A: Thanks for asking this question. There are two ways of incorporating amendment into the soil. One is to lay it on the surface and the other is to dig up the soil around the roots and put it more deeply.

I prefer to lay it on the surface, cover it with mulch and periodically moisten the mulch to help the amendment or compost decompose. Periodically means to hand water the mulch area at the base of the plant perhaps once or twice a month. That is all.

The amendment closest to drip emitters or other sources of water will decompose rapidly. The other areas that are hand watered will decompose more slowly. This will release nutrients to the upper surface of the soil.

It is important to remember that the vast majority of roots that take up nutrients are close to the soil surface; they are not found deep. A good rule of thumb is to think of the depth of the root system divided into quarters or fourths.

Most woody landscape plants will have the majority of their roots in the upper 18 to 24 inches. Let’s assume 18 inches. Divide the 18 inches into fourths leaving a little more than 4 inches for each quarter. The top 4 inches of soil, provided it has been irrigated regularly, will contain about 40 percent of the plant’s entire root system. The next 4 inches will contain about 30 percent of the root system. We can see that 70 percent of the plant’s root system is in the top 8 inches of soil.

This is primarily where the plant will feed from. By applying compost or amendments that can decompose easily on the surface and keeping it moist, the feeder roots in those top few inches will have access to those nutrients. It is not a good idea to disturb or damage this top surface area in any way.

The type of amendment that you use will impact the growth of the plant. Compost is high in nitrogen and you will see plants respond to compost vigorously with new shoots and larger leaves. Mulch has very little nitrogen in it and plants will not respond visually to applications of mulch.

But the soil will respond to applications of mulch by turning from light brown or even gray to a rich, dark chocolate brown in a matter of one year. As you continue to add mulch in successive years, the change in the soil will get even more dramatic. The plant will begin to shift more of its feeder roots, which take up nutrients, into this rich surface area.

You will see worms in the soil, all sorts of different insects and mushrooms on occasion, which help to break down wood mulch; your plant will respond accordingly. This is my preferred way of altering the soil and improving plant growth in desert soils.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.